7 minute read
The Chemistry Behind Composite Fillings: A Brief Overview
Ayush Surendran
Introduction
The bacterium in mouths respire to produce lactate - an acid that slowly erodes the hardy enamel layer. Soon, they begin to concoct a biofilm, which over time engenders the demineralisation of the tooth and accumulation of this film in cavities, leading to infections[1]. Bacterium and their metabolic activities have caused annual dental visits to soar, with nearly 80% of these visits resulting in a filling procedure according to a 2021 United Kingdom statistic - thus depicting fillings as the most commonplace restorative treatments against caries, cavities and decay[2] .
Approaching the 20th century, both amalgam and composite fillings were used widely by orthodontists. However upon the Amalgam Filling Controversies, sparked by research from Dr Alfred Stock (1926), The University of Calgary (1984), The University of Saskatchewan (2009) and human autopsy studies, it was suggested that the mercury base in amalgam could vaporise and ionise into the blood- causing adverse effects to the elderly and pregnant[3]. Moreover, the corrosion of amalgam in the oral space posed a serious problem as it dramatically reduced the filling’s ability to prevent further infections[4] .
Composites on the other hand, have become the most predominant filling form to reinforce teeth, and due to its various chemical properties and adjustability, are used in earnest in the current day.
The production of composite fillings
Although varying definitions of composite fillings seem to simplify its chemistry, its structure at an atomic level proves to be complex. At its core, an addition polymerisation of Bis-GMA monomers are the main reaction required to generate a compact substance.
Bis-GMA - a weighty and viscous molecule - is first diluted with the lower molecular weight TEGDMA molecule. The ratio of Bis-GMA to TEGDMA must however be maintained at a 3:4 ratio[5] in order to counteract the common problem of shrinkage- a phenomenon owing to contractions of the polymer after being bonded to the small cavity walls, which could lead to the formation of fissures or fractures in the tooth structure[6] .
In order to increase the composite’s strength, durability and optical attraction, inorganic fillerstypically silicon derivatives - are added in the second stage of synthesis. Nowadays, silicon derivatives are composed of extremely small nanosilica particles, which improve the tooth’s finish and reduce abrasion against adjacent teeth[7]. Apart from the aforementioned properties of fillers, they incur radioopacity, a feature paramount in distinguishing the filling from the tooth in an X ray, ergo allowing prompt detection of secondary caries[8]
To bind the organic and inorganic phases, a coupling reaction occurs using Silane offshoots. Silane’s contain two reactive groups, where typically vinyl/amino groups bond to the Bis-GMA/TEGDMA complex, and where alkoxy groups bind to the inorganic elements[9]. Once bound, a hydroquinone and 2-hydroxy-4-methoxy benzophenone are merged into the mixture to form a primary resin that is more resistant to colour changes and has longer shelf life[7]
The curing process
The imperative polymerisation reactions owe allegiance to the self and light curing stages. In the former, aromatic tertiary amines and benzoyl peroxide act as accelerators and initiators respectively, with the amine giving rise to the formation of free radicals from benzoyl peroxide - which attack the electrons in the carbon double bonds in Bis-GMA/TEGDMA molecules - forcing the methacrylate groups to amalgamate, forming a redox addition polymer[10]
Conversely in the latter, camphorquinone - a photoactivator - progenates the warring free radicals through the action of blue light. This type of light is most apt as the wavelength range (400-500 nanometers) of blue light nearly perfectly correlates to the absorption range of camphorquinone, ergo blue light photons are more readily absorbed by the activator in comparison to other compounds[11]
With the curing processes used individually or in tandem, the previously liquid resin solidifies during polymerisation to create the final filling enclosed within the cavity space.
Sketches of the chair configuration of camphorquinone and its absorption spectrum [19,20]
Conclusion
Composite fillings have remained an orthodontist’s weapon in an age of poorer overall oral health and changing reasons behind restorations. Stronger, safer and more optically attractive in comparison to amalgams, Composites have the potential to raise standards in these categories through the recent introduction of spiro-ortho carbonates (SOC’s) and ethylene glycol members that reduce the double bonds to increase mean molecular weight or expand in order to reduce the shrinkage postpolymerisation [7,12]. TTEMA as a substitute for Bis-GMA has already resulted in a 10% decrease in shrinkage when paired with TEGDMA[13]. Adjacently, a myriad of mixture ratios and nanoparticle members (such as nano-zirconium oxide) are under testing to better suit the needs of the patient in relation to the anterior and posterior regions of the jaw[14]. Through ongoing research on alternative chemicals and stoichiometry to be used in resin manufacture, it is hoped to increase efficiency and efficacy of composite filling treatments in the remnants of the 21st century.
References
1) Kidd, E. A. M. (2004). How “clean” must a cavity be before restoration? https://karger.com/cre/article-pdf/38/3/305/2498368/000077770.pdf
2) Adult Oral Health Survey 2021: Self-reported health of teeth and gums. GOV.UK. (2024, January 25). https://www.gov.uk/government/statistics/adult-oral-health-survey-2021/ adult-oral-health-survey-2021-self-reported-health-of-teeth-and-gums
3) Zyvoloski, T., Faraj, Z., Qadi, S., & Sonaty, M. (n.d.). A history of the amalgam filling controversy: What you need to know . https://www.studiozdental.com/blog/a-history-of-theamalgam-filling-controversy/
4) M, F., & V, M. (2004). A review on dental amalgam corrosion and its consequences. A Review on Dental Amalgam Corrosion and Its Consequences . https://www.ericdavisdental.com/biological-dentistry/symptoms-of-toxicity/galvanism/ Galvanism-2.pdf
5) Barron, D. J., Venhoven, B. A. M., Eick, J. D., Maffezzoli, A., Anseth, K. S., Cook, W. D., Kalipcilar, B., Vaidyanathan, J., Soh, S. K., Ballard, M. J., Zhu, S., Lovell, L. G., Taira, M., & Hansel, C. (2001, May 1). Primary cyclization in the polymerization of bis-GMA and TEGDMA: A modeling approach to understanding the cure of dental resins. Dental Materials. https://www.sciencedirect.com/science/article/abs/pii/S0109564100000750
6) Bowen, R. L., Calheiros, F. C., Braga, R. R., Labella, R., Vaidyanathan, J., Silikas, N., Feilzer, A. J., Lim, B.-S., Miguel, A., Watts, D. C., Sakaguchi, R. L., Lovell, L. G., Asmussen, E., Dauvillier, B. S., Chen, H. Y., Condon, J. R., Lu, H., Alster, D., Kinomoto, Y., … Bouschlicher, M. R. (2005, October 8). Factors involved in the development of polymerization shrinkage stress in resin-composites: A systematic review. Dental Materials. https://www.sciencedirect.com/science/article/abs/pii/S0109564105001685 http://www.medicinaoral.com/medoralfree01/v11i2/medoralv11i2p215.pdf
7) García A, H., Lozano, M., Vila J, C., Escribano A, B., & Galve P, F. (2006, January 29). Composite resins. A review of the materials and clinical indications. Medicinaoral.
8) Lachowski, K. M., Botta, S. B., Lascala, C. A., Matos, A. B., & Sobral, M. A. P. (2013, February). Study of the radio-opacity of base and liner dental materials using a digital radiography system. Dento maxillo facial radiology. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3699023/
9) Özcan, M., Darvell, B. W., Kolodney, H., Gbureck, U., Lung, C. Y. K., Hooshmand, T., Aboushelib, M. N., Akova, T., Jevnikar, P., Piascik, J. R., Matinlinna, J. P., Oyagüe, R. C., Kitayama, S., Kim, S. H., McDonough, W. G., Debnath, S., Goracci, C., Ferracane, J. L., Nergiz, I., … Fischer, T. E. (2012, March 14). Aspects of silane coupling agents and surface conditioning in Dentistry: An overview. Dental Materials. https://www.sciencedirect.com/science/article/abs/pii/S010956411200053X
10) Brauer, G. M. (1981, January 1). Initiator-accelerator systems for acrylic resins and Composites. SpringerLink. https://link.springer.com/chapter/10.1007/978-1-4757-95103_29
11) Kowalska, A., Sokolowski, J., & Bociong, K. (2021, February 2). The photoinitiators used in resin based dental composite-a review and future perspectives. MDPI. https://www.mdpi.com/2073-4360/13/3/470
12) Moon, E. J., Lee, J. Y., Kim, C. K., & Cho, B. H. (2005, January 27). Dental restorative composites containing 2,2-bis-[4-(2-hydroxy-3-methacryloyloxy propoxy) phenyl] propane derivatives and spiro orthocarbonates. Journey of Biomedical Materials Research. https://onlinelibrary.wiley.com/doi/abs/10.1002/jbm.b.30222
13) Labella, R., Matsukawa, S., Tarumi, H., Zamora, M. P., Nishiyama, N., & Stansbury, J. W. (2001, December 19). Development of a new photocurable composite resin with reduced curing shrinkage. Dental Materials. https://www.sciencedirect.com/science/article/abs/pii/S0109564101000392
14) Hu, C., Sun, J., Long, C., Wu, L., Zhou, C., & Zhang, X. (2019, January 1). Synthesis of nano zirconium oxide and its application in dentistry. De Gruyter. https://www.degruyter.com/document/doi/10.1515/ntrev-2019-0035/html
15) https://crystaldentalcenters.com/blog/amalgam-vs-composite-fillings-whats-the-bestoption-for-you/
16) https://pocketdentistry.com/radiographic-appearance-of-dental-tissues-and-materials/
17) http://www.powerchemical.net/coupling1.htm
18) https://www.researchgate.net/figure/Structural-formula-of-Bis-GMA-and-TEGMA-Ryc-1Wzory-strukturalne-Bis-GMA-i-TEGMA_fig1_263014522
19) https://pubchem.ncbi.nlm.nih.gov/compound/Camphorquinone
20) https://www.researchgate.net/figure/Absorption-spectrum-of-camphorquinone-in-relationto-the-different-emission-spectra-of_fig2_328126011
21) https://link.springer.com/article/10.1007/s13233-010-1005-z
